Previously, various magnetic recording media have been proposed in which the composition of backcoat layers is specified.
For example, proposed are "a magnetic recording medium comprising a magnetic recording layer formed on one side of a non-magnetic support and a backcoat layer formed on the other side thereof, in which said backcoat layer contains 40 parts to 180 parts by weight of carbon black per 100 parts by weight of resin binder, said carbon black having an oil absorption of 50 cc/100 g to 70 cc/100 g and a pH of 6.0 to 12" (Japanese Unexamined Patent Publication No. 4-44626); "a magnetic recording medium comprising a magnetic recording layer provided on a surface side of a non-magnetic support and a backcoat layer provided on a back side thereof, in which said backcoat layer contains finely divided carbon black having an average particle size of 0.01 .mu.m to 0.035 .mu.m, coarsely divided carbon black having an average particle size of 0.08 .mu.m to 0.5 .mu.m, and at least one kind of powder selected from a graphite particle powder and a tetrafluoroethylene resin powder" (Japanese Unexamined Patent Publication No. 64-50230); "a magnetic recording medium comprising a magnetic layer formed on a surface side of a non-magnetic support and a backcoat layer formed on a back side thereof, in which said backcoat layer contains finely divided carbon black having an average particle size of 0.01 .mu.m to 0.035 .mu.m and graphitized carbon black having an average particle size of 0.1 .mu.m to 0.5 .mu.m" (Japanese Unexamined Patent Publication No. 1-134717); "a magnetic recording medium having a magnetic layer on one side of a non-magnetic support and a back layer on the other side thereof, said back layer having a thickness of 2 .mu.m or more and containing carbon black, a binder and optionally an inorganic powder, in which said back layer contains at least tow kinds of finely divided carbon black having an average particle size of 10 m.mu. to 30 m.mu. and coarsely divided carbon black having an average particle size of 150 m.mu. to 500 m.mu., and the weight ratio (P/B) of the sum (P) of finely divided carbon black, coarsely divided carbon black and the inorganic powder to a binder (B) ranges from 1/0.5 to 1/3.5" (Japanese Examined Patent Publication No. 4-81256); and "a magnetic recording medium comprising a magnetic layer provided on a non-magnetic support, said magnetic layer comprising a binder, and a ferromagnetic powder and carbon black dispersed therein, in which said binder contains a copolymer having amine-modified vinyl units and acidic functional group-containing vinyl units, and said carbon black has a specific surface area of 90 m.sup.2 /g to 330 m.sup.2 /g and a DBP oil absorption of 45 ml/100 g to 120 ml/100 g" (Japanese Unexamined Patent Publication No.7-153055).
However, the conventional magnetic recording media have various drawbacks. For example, Japanese Unexamined Patent Publication No. 4-44624 discloses that the backcoat layer of the magnetic recording medium described therein is good in running durability and decreases drop out. However, carbon black having an oil absorption of 50 cc/100 g to 70 cc/100 g can not be sufficiently dispersed, with aggregates left, unless it is dispersed at a P/B ratio of 2.5/1 or more, that is, a considerably higher P/B ratio than the ratio desired finally.
The Japanese Unexamined Patent Publication No. 64-50230 describes that the backcoat layer of the magnetic recording medium described therein has a satisfactory surface smoothness and a low friction coefficient against both guide pins made of metal and plastic. However, within this range of the weight ratio of finely divided carbon black/coarsely divided carbon black, the weight ratio of coarsely divided carbon black is too high, so that coarse projections increase on the backcoat layer to cause transcription to the magnetic layer, resulting in reduction in S/N ratio.
Further, Japanese Unexamined Patent Publication No. 1-134717 describes that the backcoat layer of the magnetic recording medium described therein has a satisfactory surface smoothness and a low friction coefficient against both guide pins made of metal and plastic. However, within this range of the weight ratio of finely divided carbon black/coarsely divided graphitized carbon black, the weight ratio of coarsely divided graphitized carbon black is too high, so that coarse projections increase on the backcoat layer to cause offset to the magnetic layer, resulting in reduction in S/N ratio.
With respect to the backcoat layer of the magnetic recording medium described in the Japanese Examined Patent Publication No. 4-81256, a process for efficiently preparing a paint is not described in detail, and the binders described in the examples fail to assure sufficient S/N characteristics and durability for high density recording now required.
Further, Japanese Unexamined Patent Publication No. 7-153055 describes that the magnetic recording medium described therein has the magnetic layer in which carbon black having a specific surface area of 90 m.sup.2 /g to 330 m.sup.2 /g and a DBP oil absorption of 45 ml/100 g to 120 ml/100 g, and a ferromagnetic powder are dispersed into the binder having amine-modified vinyl units and acidic functional group-modified vinyl units, and therefore is excellent in antistatic effect and electromagnetic characteristics. However, the process for preparing the paint differs from that of the present invention.
Although the preparation of backcoat paints is generally carried out by means of medium-agitation type dispersing devices such as ball mills, sand grinder mills and attriter mills, uniform dispersion can not be achieved in a short time, resulting in poor production efficiency. Then, when composition components are kneaded by means of pressure kneaders, followed by dispersing thereof in sand grinder mills, efficient production becomes possible. In this case, however, the letdown (LD) is conducted, for converting the optimum dispersing viscosity for dispersing in the sand grinder mill and the weight ratio of a pigment (carbon black) to a binder (hereinafter abbreviated as "P/B") to the optimum coating viscosity and the final P/B for coating. In this case, however, the LD shock is liable to occur. In spite of sufficient dispersion in the dispersion step with much effort, the surface properties of the backcoat layer are often deteriorated by a reduction in gloss and generation of aggregates caused by the LD shock. Accordingly, it is not said that the stability of the paint is sufficient. The backcoat layer having poor surface properties causes offset from the backcoat layer to the magnetic layer, resulting in a reduction in SIN.
On the other hand, the smoothing of a surface of the backcoat layer increases the torque against tape guides incorporated in a VHS cassette when the medium starts to run, which enhances the probability of the occurrence of troubles such as poor rewinding.